1. Field of the Invention
This invention relates generally to liquid storage tanks, and more particularly to a device for accurately determining, from the top of the tank, the level of the liquid in the tank.
2. Description of the Prior Art
In businesses whose activities include storage of liquids in some kind of tank, it is at least of interest, and usually necessary, to have some means to determine the level of the liquid in a tank. A rudimentary means used in some cases is a sight glass in the wall or connected to the wall of the tank. In other cases, where access to the side of a tank is impossible, or inconvenient or impractical, a measuring stick is inserted through an opening in the top of the tank to measure the liquid level. The stick may be inserted to the bottom of the tank and then withdrawn, to note how much of the stick has the liquid on it. The stick may or may not be graduated. A more recent and refined approach is to insert into the liquid, a tube having a closed lower end, a graduated stem inside the tube, a float encircling the tube and having a magnetic coupling to an armature at the bottom of the stem. When the assembly is mounted to a top opening in a tank, the float will cause the measuring stem to settle at a level controlled by the float and whereby measurements on the stem, relative to the tube top, indicate the liquid level.
Ultrasonic ranging is well known for measuring distances. However, it is significantly affected by air movement and temperature changes. Therefore, to my knowledge, it has not been useful to detect the level of liquid in a tank in which there is any air movement, or a temperature variation due to depth or ambient conditions. A classic example of such environment is a railroad tank car. But it has remained important to be able to accurately determine the level of liquid in a tank car through a top access opening and in a convenient way. The present invention is addressed to this need.